THE UNIVERSITY OF CALGARY Multicomponent Seismic Data Interpretation Susan L.M. Miller A THESIS SUBMTED TO THE FACUL,TY OF GRGDUATE STUDIES IN PARTIAL FULFlLMENT OF THE REQUEEMENTS FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF GEOLOGY AND GEOPHYSICS CALGARY,ALBERTA DECEMBER 1996 O Susan L.M. Miller 1996 National Library Bibliothitque nationale du Canada Acquisitions and Acquisitions et Bibliographic Sewrvlces senhces bibliogr-hipues 395 welri~tnret 395, rue weaingtm OctawaON K1AW OftawaON KIAON4 Canada Canada The author has granted a now L'auteur a accord6 melicence non exclusive licence ailowing the exclusive permettant a la National Li'brary of Canada to Biblioth-e nationale du Canada de reproduce, loan, disttl'bute or sell repfoduire, pr*, distniuer ou copies of his/her thesis by any means vendre des copies de sa th&e de and in any form or format, making this thesis available to interested fonne que ce soit pour mettre des persons. exemplaires de cette these B la disposition des personnes interedes. The author retains ownership of the L'auteur conserve la propridte du copyright in hismer thesis. Neither droit d'auteur cpipmt6ge sa thh. Ni the thesis nor substantial extracts la these ni des extmh substantiels de fkom it may be printed or otherwise celle-ci ne doivent Stre imprim& ou reproduced with the author's autrement reproduits sans son permission. Abstract A procedure is developed for the coupled interpretation of multicornponent (P-P and P-S) seismic data, and is illustrated using two 3C-2D seismic datasets from Alberta, Canada. In both cases, numerical modelling studies were used to assist the interpretation. The principal objective of the Lousana survey was to differentiate reservoir dolomite from tight anhydrite within the Nislcu Formation using seismic methods. VpNs analysis of two intervals which contained the target mapped a decrease in VpNs coincident with productive wells. The second survey, from the Blackfoot Field, targeted incised-valley sandstones in the Lower Cretaceous. The exploration gods were to seismically delineate the edges of an incised valley and to distinguish between sandstone and shale valley-fill sediments. The valley edges were defined by P-P and P-S seismic character changes. Within the incised valley, a decrease in VpNs was interpreted to indicate sandstone sediments, while increasing VpNs toward the northwest indicated increasing shaliness within the incised valley. Acknowledgements Many people helped with the work presented in this thesis. I would like to thank my supervisor, Don Lawton, for his guidance, support, and good humour throughout the course of this work. The Lousana work was made possible by the generous donation of the seismic data by Unocal Canada Ltd. Andrea Bell, formerly of Norcen, provided background on the geology of the area Dr. Mark Harrison processed the Lousana data and provided helpful insights. Dr. Robert Stewart and Mr. Ken Szata also contributed ideas and advice regarding the work on the Lousana Field. Many people at PanCanadian freely shared their knowledge about the geology and geophysics of the Blackfoot Field: Andre Polity lo, Ian Shook, Bill Goodway, Dave Cooper, and Garth Syhlonyk. Many thanks to Dr. Gary Margrave and Ms. Evsen Aydemir for their assistance with the Blackfoot study and also for lots of laughs along the way. Kudos to Henry Bland and Darren Foltinek, who could always figure out a way to make the software and hardware work, and were also great companions. Thanks to all of the CREWES people, who enriched my university experience and provided great memories (and some really fumy stones). Thanks also to the Sponsors of the CXEWES Project for financial support and technical advice. I am very grateful to my family for their unflagging support. My parents instilled in me the belief that I could accomplish whatever 1 set out to do, and provided emotional and financial support when it was needed. Finally, and most importantly, I would like to thank my son. Rhys, who never once complained about being alone or supperless on the many late nights and working weekends. Instead he offered cheerful support and encouragement, which made my task a great deal easier. List of Tables Table 2.1 Field acquisition and recording parameters for the Lousana survey ..............11 Table 2.2 Rock property values used for numerical models.................................. -32 Table 3.1 FieId acquisition and recording parameters for the Blackfoot survey ...........-50 FIG . 3.1 6 Interpretation of the P-P seismic data.. ..............................................67 FIG . 3 .L 7 Interpretation of the PS seismic data .............................................. -69 FIG . 3.18 P-P and P-S isochmns for the Viking to Shunda interval ......................... 70 FIG . 3.19 VpNs values calculated for the Viking to Shunda interval ....................... -71 FIG . 3.20 VpNs versus gamma values in the Glauconitic Formation...................... -72 F[G . 3.2 L Vs versus Vp in the Glauconitic Formation .......................................73 Glossary of ScienMic Terms 3-C seismic: A seismic survey which uses a conventional energy source and is recorded on 3-C geophones. 3-C Geophone: Seismic recording device with three orthogonal (or trigonal Galperin) coils which respond to ground motion in three orthogonal directions. 3C-2D seismic survey: Two-dimensional seismic survey recorded on 3-C geophones. Bandpass filter: A filter which allows the passage of certain frequency components and attenuates others. Dipole sonic log: Sonic logging tool which uses a dipole source to deform the borehole and subsequently records P- and 9wave transit times. Groundroll: Surface wave which propagates by retrograde elliptical particle motion. Characterized by high amplitude, low frequency, and low velocity. Imhron: The time interval between two interpreted seismic horizons. Mode: Refers to type of wave propagation, e.g. compressional mode or shear mode. Multicomponent seismic: Seismic data acquired with more than one source and/or receiver mode; in this thesis, refers to a conventional source and 3-C recording. P wave : Pressure, compressional, or longitudinal elastic body wave; direction of propagation is parallel to particle motion. P-P seismic: Seismic waves travelling down as P waves, reflecting from an interface, and travelling up as P waves. In this thesis, waves recorded on the vertical component of the geophone are assumed to be largely P-P mode. P-S seismic: Seismic waves travelling down as P waves, reflecting and converting at an interface, and travelling up as S waves. In this thesis, waves recorded on the radial component of the geophone are assumed to be largely PS mode. Radial component: Horizontal geophone coil which responds to horizontal ground motion in line with the source-receiver azimuth. SP: Shot point, i.e. station number for seismic source location. Statics: Time-shift correction applied to seismic data to compensate for the velocity effect of near-surface stratigraphy by adjusting the traces to a common datum. S wave: Shear elastic body wave; direction of propagation is perpendicular to particle motion. Synthetic seismogram: An artificial seismic record made by, in the zerooffset case. convolving a wavelet with a reflectivity series. In the offset case, a layered model is ray- traced using a chosen geometry and an amficid shot gather is computed, which can also be stacked. Tramverse component: Horizontal geophone coil which responds to horizontal ground motion orthogonal to the source -receiver azimuth. Vertical component: Vertical geophone coil which responds to vertical ground motion. Vp: P-wave velocity VpNs: Ratio of P-wave velocity to S-wave velocity Vs: Swave velocity Chapter 1 - Introduction 1.1 Background Coupled P-P and P-S seismic analysis increases confidence in interpretation, provides additional measurements for imaging the subsurface, and gives rock property estimates. Supplementary P-S data from 3component (3-C) seismic recordings is obtained for a relatively small additional cost, as conventional sources and nceiver geometries are employed. Compressional (P) waves impinging on an interface at non-normal incidence are partitioned into transmitted and reflected P and shear (9 waves. Significant energy is converted to S waves which, in the absence of azimuthal anisotropy, will be recorded primarily on the radial (inline horizontal) component of the receiver. Due to the difference in travel path, wavelength, and reflectivity, P-S seismic sections may exhibit geologically simcant changes in amplitude or character of events which are not apparent on conventional P-P sections. Horizons may be better imaged on one or the other of the sections because of different multiple paths and wavelet interference effefts such as tuning. It is helpfbl to have another seismic section to work with in areas where the data quality is poor or the interpretation is unclear. Through the analysis of multicomponent seismic data, important rock properties such as VpNs (or similarly, Poisson's ratio) can be extracted. This elastic parameter can improve predictions about mineralogy, porosity, and reservoir fluid type (e.g. Pickett, 1963; Tatham, 1982; Rafavich et al., 1984; Miller and Stewart, 1990). Compressional seismic velocity alone is not a good lithology indicator because of the overlap in Vp for various rock types. The additional information provided by Vs can reduce the ambiguity involved in interpretation. Pickett (1963) demonstrated the potential of VpNs as a lithology indicator through his laboratory research. Using core measurements he determined VpNs values of 1.9 for limestone, 1.8 for dolomite, 1.7 for calcareous sandstone, and 1.6 for clean sandstone. Subsequent research has generally confirmed these values and has also indicated that VpNs in mixed lithologies varies linearly between the VpNs values of the end members (Nations, 1974; Kithas, 1976; Eastwood and Castagna, 1983; Rafavich et al., 1984; Wilkens et al., 1984; Castagna et al., 1985). Goldberg and Gaat (1988) studied full-waveform sonic log data in a limestondshale sequence and found VpNs effective at identifying limestonelshale boundaries, but ineffective at identifying fracturing in the limestone.